As it is known, the isotactic polypropylene, though being endowed with an exceptional combination of excellent properties, is affected by the drawback of possessing an insufficient impact resistance at relatively low temperatures, specially below its glass transition temperature (Tg).
According to the teaching of the prior art, it is possible to obviate the said drawback without sensibly affecting the other polymer properties, by properly adding rubbers and polyethylene to the polypropylene.
For example, the U.S. Pat. No. 4,245,062 discloses a process for producing a blend of polypropylene and two different propylene-ethylene copolymers, one of said copolymers substantially forming the rubbery phase. The propylene polymers thus obtained have good impact resistance at low temperature.
In U.S. Pat. No. 4,521,566 a polypropylene composition is disclosed having the following composition:                74 to 91% of crystalline isotactic polypropylene,        6.7 to 19% of an amorphous copolymeric fraction of ethylene and propylene and        1.5 to 8.5% of crystalline polyethylene containing from 50 to 98 of ethylene and exhibiting a crystalline of polyethylene type.        
The U.S. Pat. No. 4,473,687 discloses polypropylene moulding compositions of polypropylene, ethylene-propylene copolymer and polyethylene in different proportions, said composition having high hardness and increased impact strength.
In U.S. Pat. No. 4,734,459 a polypropylene composition having good whitening resistance is disclosed. According to the teaching of the said prior art document, it is possible to improve whitening resistance by replacing the ethylene-propylene copolymer rubber with an ethylene-butene-1 copolymer rubber.
In the European patent application EP-A-1 236 769 (Borealis) an heterophasic propylene composition is disclosed with improved balance of impact stiffness and stress whitening resistance comprising a crystalline propylene polymer matrix (i) an elastomeric component (ii) and an ethylene copolymer plastomer. (iii). The elastomeric component (ii) is a propylene/ethylene copolymer (C3/C2) and the ethylene copolymer plastomer (iii) is a metallocene catalyst derived plastomer (Exact C2C4 and C2C8), in the examples.
The international patent application WO2006/067023 A1 discloses polyolefin compositions obtained by sequential polymerization having:    a) 50-77% of a crystalline propylene polymer having high isotacticity;    b) 13-28% of an elastomeric copolymer of ethylene and propylene, partially soluble in xylene at room temperature (25° C.); and    c) 10-22% of polyethylene having an intrinsic viscosity value ranging from 1 to 3 dl/g and optionally containing recurring units deriving from propylene (copolymerized propylene) in amounts up to lower than 10% (HDPE).
The international patent application WO2010/081753 A1 discloses polyolefin compositions comprising from 30% to 90% by weight of a propylene polymer composition (A) comprising a crystalline propylene homopolymer, or a copolymer of propylene containing 3% or less of derived units of ethylene or C4-C10 α-olefin(s) or of combinations thereof, and an elastomeric copolymer of ethylene containing from 40% to 70% by weight, of units derived from propylene or C4-C10 α-olefin(s) or of combinations thereof; and B) from 10% to 70% by weight of a multimodal ethylene copolymer with alfa olefins as comonomer of formula CH2═CHT wherein T is a C3-C10 alkyl group, having density (with ISO 1183) comprised in the range from 0.850 to 0.935 g/cm3.